Among the technological problems to be solved before realization of the attractive new generation of optical fiber communication called "coherent optical communication", the use of the nominal single-mode optical fiber as the transmission medium presents a major difficulty. This is because, in a conventional optical fiber with circular core and claddings, there exist two nearly degenerate orthogonally polarized modes, so that an incident linearly polarized mode will assume an unpredictable polarization after trasversing a distance along a practical optical fiber transmission line whose property is always variably perturbed. This unpredictability of the polarization orientation is most annoying in a coherent communication system which requires polarization matching of the signal with the local oscillator of the receiver.
To overcome the difficulty, great efforts have been made during recent years along two different lines of thought and technological approaches.
The first approach is a continual attempt aimed at using high-birefringent fiber, instead of conventional fiber, for the entire transmission line of a coherent optical communication system. Remarkable progress, notably in the making of relatively long Panda fiber, has been achieved in this direction. Y. Sasaki et al, 26 km-long polarization-maintaining fibre, Electron. Lett., vol. 23, No. 3, pp. 127-128 (1987). T. Kimura, Coherent optical fiber transmission, J. Lightwave Tech. vol. LT-5, No. 4, pp. 414-428 (1987). For practical utilization of high-birefringent fiber of one or the other version in a long-haul coherent optical fiber transmission line, the high-birefringent fiber is required to have over-all transmission characteristics as good as the well-developed conventional optical fibers. In particular, low-loss for the propagating mode of the desired polarization and small coupling between this mode and the mode of the undesired polarization is required. Additionally and sometimes more importantly, the problem of cost is a deterministic factor to judge whether or not it is favorable to use high-birefringent fiber for the entire long-haul line.
The second approach takes advantage of using the well-developed conventional low-loss single mode optical fibers as the transmission medium without any structural modifications, while incorporating at the far end of the long line a polarization-control device to control the unpredictable polarization. The polarization-control device can be used not only in future coherent optical communication lines, but also at terminals of existing optical fiber transmission lines for upgrading the communication characteristics. This second technological approach was probably induced, or encouraged, by the well-known BTRL's experiments, which indicated that the output polarization from a conventional long fiber, though unpredictable, is relatively stable, with a time constant in the order of hours, so that the changing polarization can be controlled by the currently available electronics-optics techniques. D. W. Smith et al, Polarization stability requirements for coherent optical fiber transmission systems, OPTICAL WAVEGUIDE SCIENCES, Proceedings of International Symposium, Martinus Nijhoff Publishers, the Hague/Boston/Lancaster, Jun. 1983, pp. 133-156. Such polarization-control devices fall into the category of active scheme because all of them require some form of active opto-electronic circuitry. Also the higher the required control performance, the more complicated and sophisticated the over-all electronics/optics generally will be. R. Ulrich, Polarization stabilization on single-mode fiber, Appl. Phys. Lett., vol 35, No. 11, pp. 840-842 (1979). T. Okoshi, Polarization control schemes for heterodyne-homodyne optical fiber communications, J. Lightwave Tech., vol. Lt-3, No. 6, pp. 1232-1237 (1985). While trial systems following this active approach have been used and advanced during recent years, it doesn't seem likely to see in the near future the realization of an absolutely manual-free polarization control device for coherent communication use.
The present invention, which is based on an analytic prediction of the super-mode theory, represents a new approach to solving the polarization-control Huang Hung-chia, Weak coupling theory of optical fiber and film waveguides, Radio Science, vol. 16, No. 4, pp. 495-499 (1981). Huang Hung-chia, COUPLED MODES AND NONIDEAL WAVEGUIDES, Microwave Research Institute (MRI), Polytechnic Institute of New York, Nov. 1981. Instead of an active circuitry, a purely passive fiber-optic device is used at the far end of a conventional optical transmission line to automatically change an unpredictable polarization orientation to a specified orientation. The price paid for such an all-passive fiber-optic device is a power penalty of 3 dB. From the power-budget point of view, this generally will not pose a problem, as coherent optical communication systems are capable of improving the sensitivity of the present-day direct-detection receiver by 15-20 dB. In view of the current line of thought that coherent optical systems may prove useful also in local networks, where the primary interest is in the multiplexing possibility, while providing a useful increase in power budget, the present invention adopting the passive-device approach is apparently superior for its low cost and engineering simplicity. The passive polarization-control device also possesses the advantage of possible afterwards upgrading of the terminal devices. Such a feature is preferred more than the all high-birefringent fiber approach.
Advantages of the present invention, most of which are due to the passive nature of the device, are summarized below:
(1) small size and light weight; PA1 (2) structural simplicity; PA1 (3) operational dependability; PA1 (4) low cost; PA1 (5) no limitation on finite controllable-range of the polarization; PA1 (6) no limitation on the time rate of the randomly-varying input polarization; and PA1 (7) long life-time.